This invention relates to ultrasonic imaging apparatus and particularly to ultrasonic diagnosis apparatus for forming tomographic images of internal elements of a living body.
In ultrasonic diagnosis apparatus, acoustic beams are emitted into a subject, echoes corresponding to the direction of the beam positions are received, scanning lines corresponding to the beam direction with the echoes are modulated to conform them to a display apparatus, and a tomographic image of the subject is obtained by using the acoustic beams of a plurality of beam directions.
For example, ultrasonic diagnosis apparatus techniques for causing lateral resolution to become more precise include the following:
(i) Apertures of emitted acoustic beams are caused to change corresponding to the required depth being diagnosed, that is, in case the required depth is relatively shallow the beam aperture is caused to be small to improve the lateral resolution. In case the required depth is deep, the beam aperture is caused to be relatively large to provide the scope of the long distance acoustic field, which narrows as it penetrates, and checking deterioration of the lateral resolution because of the extensions of the beams in the long distance acoustic field. Hence, the smaller the beam aperture is, the shallower the scope of the short distance acoustic field is and the larger the beam aperture is, the deeper the scope of the short distance acoustic field is; PA1 (ii). Focusing the emitted acoustic beams causes the lateral resolution to become better defined in the vicinity of the required depth being diagnosed; PA1 (iii). The received beam aperture in echo-receiving time is caused to be small in case of the short distance acoustic field and to be large in case of the long distance acoustic field to define the lateral resolution, more precisely, and; PA1 (iv). The received focusing point in echo-receiving time is caused to shift corresponding to the reflective point of the received echo to provide precise lateral resolution.
These techniques have been mainly realized in a so-called electronic scan type of ultrasonic apparatus and, in the apparatus of the invention, are put to practical use by properly combining these techniques and by adding other techniques to them, as will be described.
An example of conventional concrete techniques for improving the lateral resolution with respect to the direction of the diagnostic depth follows. In the ordinary ultrasonic diagnosis apparatus, a train of repeating rate pulses having identical wave form, for example, as shown in FIG. 1(a), is set up, and a train of acoustic pulses is emitted on the timing of the rate pulses. An example of echo waves corresponding to the acoustic pulses is shown in FIG. 1(b). Referring to FIG. 2, there are carried out the transmission and the reception of the acoustic pulse beams emitted by, for example, m.sub.1 elements (in the first case twelve) of unit transducers UT of an electronic scan probe EP. The grouped emissions of the linearly disposed unit transducers are designated Tla, in accordance with the rate pulse RPla, of FIG. 1(a).
In the first example of FIG. 2, a focus is set at the relatively long distance point F1a on an imaginary line a1 extending from the probe EP to focus the transmitting and receiving acoustic beams. The determination of the focal point gives the time difference in accordance with particular patterns as to the timing for driving the unit transducers and as to the timing for processing the received echo signal. As the result, the lateral resolution near the focus F1a (the focus position is practically coincident with the position of the center of the beam diameter), namely in the comparatively long distance region, becomes more precise.
Synchronizing the transmission and reception of acoustic pulse beams with the next rate pulse RP1b is carried out with m.sub.2 elements (in this case eight) which are less than m.sub.1 as shown with T1b of the unit transducers UT. In this case, the focus of the transmitting and receiving beams is set at a point F1b comparatively near the probe EP on an imaginary line a2, set apart a predetermined distance from the line a1. So, the lateral resolution of the image of the portion near the focus F1b, namely the comparatively short distance region, rises.
Moreover, synchronizing with the next rate pulse RP2a, transmission and reception of the acoustic pulse beams are carried again by m.sub.1 elements of the unit transducers similar to the T1a, as shown with T2a in FIG. 2. In this case, the acoustic pulse beams focus at a point F2a on an imaginary line a3 apart a further predetermined distance from the line a2 and similar to the focus F1a in distance form the probe EP.
Likewise, synchronizing with the following rate pulse RP2b, transmission and reception of the acoustic pulse beams are carried by m.sub.2 elements of the unit transducers UT similar to the T1b as with T2b in FIG. 2. In this case, the acoustic pulse beams focus at a point F2b on an imaginary line a4 set apart another predetermined distance from the line a3 and similar to the focus F1b in distance from the probe EP. After this, the above-mentioned operations are repeated while the driving unit transducer positions and the imaginary line positions are shifted one after another as well-known in the art and shown in U.S. Pat. No. 4,161,122.
The conventional image display system in an ultrasonic diagnosis apparatus is generally composed as shown in FIG. 3, for example, in case of a linear scan. A known display apparatus to display echo images includes, for example, a brightness modulation circuit 2 for conducting to a cathode-ray tube of a display apparatus 1 video signals which include the echo data obtained by the transmission and the reception of the acoustic beams. A blanking control circuit 3 applies to the brightness modulation circuit 2 a blanking control signal to blank unnecessary components of the video signals and the returns of the linear scan in the display apparatus 1. A beam directional sweeping circuit 4 applies to the display apparatus 1 sweeping waves for the linear scan about the acoustic beam direction. A scanning line position signal generator 5 applies to the display apparatus 1 the scanning position-designating signals to designate the scanning position corresponding to each acoustic beam position in the linear scan.
FIG. 1(c) shows the form of the conventional sweeping wave generated from the beam directional sweeping circuit 4, which sweeping wave defines a saw-tooth wave synchronized with the rate pulses as shown in FIG. 1(a).
FIG. 1(d), shows the form of the scanning line position-designating signal generated from the scanning line position signal generator 5. The scanning line designating signal defines the stepping wave the level of which varies with every rate pulse.
FIG. 1(e) shows the form of the blanking control signal generated from the blanking control circuit 3, the blanking in this case being actuated when the blanking control signal is at low level L, and the unblanking signal being actuated when the signal is at high level H. As shown, the blanking control signal is in the unblanking state during the second half of the scanning line when the rate pulse RP1a or RP2a is generated, i.e. for the long distance region, and the blanking control signal is put in the unblanking state at the first half of the scanning line when the rate pulse RP1b or RP2b is generated, i.e., the comparatively short distance region.
Therefore, the display scanning line of each rate pulse in the image display, respectively, corresponds to the axis lines a1, a2 . . . of the acoustic beams shown in FIG. 2, and the blanking is actuated as to the short distance region of the display scanning line when the long distance focuses F1a, F2a, etc. are set and the blanking is actuated as to the long distance region of the display scanning line when the short distance focuses F1b, F2b, etc. are set.
Consequently, the echo images having high lateral resolution in both the long distance and the short distance are displayed, and echo images of good quality are obtained over the extensive scope along the acoustic beam direction, namely, the diagnosis depth direction. Apparatus for obtaining these results is taught in U.S. Pat. No. 4,215,584.
However, the conventional system has shortcomings. The echo image display according to the above-described system is executed as shown in FIG. 4. As is apparent, in the conventional case, each scanning line consists of a displayed region UR and an undisplayed region BR because the scanning lines contributing to the display at the short distance region are not displayed during display of the long distance region and the scanning lines contributing to the display at long distance are not displayed during display of the short distance region. As a result, the scanning line density is caused to be substantially reduced by a half. In addition, there is also the problem that, due to the discontinuities in the scanning lines between the short distance region NE and the long distance region FE, confusing and undesirable portions are caused at the border BL between them.